In variable-air-volume (VAV) diffusers, room air temperature is controlled by varying the volume of supply air which is discharged into a room. The supply air will be heated when the VAV system is in a heating mode and it will be cooled when the system is in a cooling mode. The supply air is usually provided at substantially a constant temperature for each mode. A variable-air-volume diffuser, or an upstream VAV box, is used to regulate the volume of heated or cooled supply air in order to achieve and maintain the desired room air temperature. A central building controller is used to determine whether hot supply air or cool supply air flows from the HVAC air source to the VAV diffusers or box. It is possible, of course, for only cool air or only hot air to be supplied by the system. Thus, in the tropics cool supply air may always be flowing to the VAV diffusers or box.
Three types of actuators for VAV air diffusers and/or VAV duct boxes are in wide-spread use, namely, thermally-powered actuators, pneumatically-powered actuators, and electrically-powered actuators. All three types of VAV actuators are coupled through a mechanical linkage, gear assembly levers or the like to move one or more dampers, vanes, blades, etc., (hereinafter "dampers"), in the air diffuser or the control box upstream of the air diffuser. The damper position across a diffuser discharge opening, or the supply duct in the case of a VAV box, is modulated by a thermally-powered or pneumatically-powered actuator or by an electrical motor in response to sensed room air temperature. Thus, in a heating mode as the room air temperature rises toward the desired or targeted set point temperature, the damper closes down to reduce the amount of supply air being discharged into the room. Conversely, as the room air temperature drops away from the desired or targeted set point temperature, the damper is opened to allow an increase in the amount of warm supply air discharged into the room. In a cooling mode, as the room air temperature rises and moves away from the set point temperature, the actuator opens the damper to allow more cool air to enter the room. As the room air temperature drops toward the set point in the cooling mode, the damper is closed to reduce the volume of cool air discharged into the room.
Various thermal actuators, pneumatic actuators and motor assemblies have certain operating characteristics which favor their selection for particular applications. All of these prior art VAV actuators, however, have deficiencies which, if eliminated, would enhance their performance.
Thermally-powered actuator assemblies, are described in more detail in U.S. Pat. Nos. Re. 30,953; 4,491,270; 4,509,678; 4,515,069; 4,523,713; 4,537,347; and 4,821,955, which are incorporated herein by reference. Briefly, however, such assemblies will include a containment cylinder or housing filled with a thermally expandable and contractible material, such as a wax which expands or contracts during phase changes. A piston is reciprocally mounted to the containment housing so that the outwardly displaced piston, upon heating and expansion of the wax, can be used to power movement of a damper through a mechanical linkage. Cooling contracts and causes a phase change in the wax, and the piston is drawn into the housing, usually with the aid of a biasing spring.
As will be understood, the piston can be held and the housing allowed to move to drive the damper. Other forms of thermally-powered actuators can include bi-metal elements and memory metals which change shape at selected temperatures.
The response of a typical thermally-powered sensor/actuator can be seen in FIG. 1. A piston displacement versus temperature curve 21 is shown in which the piston is fully retracted at the bottom end of the curve and is fully extended at the top end. Since it is preferable that the piston displacement versus temperature be sensitive, the linkage assembly for most diffuser thermal actuators is constructed so as not to follow displacement curve 21 into either the extreme high or extreme low ends of the curve. Thus, a linear portion of curve 21, namely, the portion defined by legs 23 and 25, can be used to drive the diffuser damper by disengaging the piston from the linkage assembly at leg 22 and by providing an overtravel mechanism at leg 24. This enables a relatively responsive or sensitive relationship to be maintained for controlling diffuser damper opening and closing. A typical actuator piston stroke used for the sensor/actuator is only about 0.1 inch, and the linkage assembly amplifies the stroke to produce longer diffuser damper displacements.
In FIG. 1, a typical sensor/actuator for cooling mode control is shown. Once the sensor senses that the room air temperature induced to flow through the diffuser is below 70.degree. F., no more cool supply air will be discharged into the room because the damper will be closed. As the sensed temperature increases (when the room begins to heat up) from below 70.degree. F., the damper does not open because the diffuser sensor/actuator is now operating on leg 22 of curve 21. The piston displacement is shown as broken line 22a, and it will be disengaged from the damper-driving linkage assembly. As the sensed temperature increases between 71.5.degree. F. to 73.degree. F., however, the thermal sensor/actuator, through the linkage assembly, begins to open the damper until it is fully open at 73.degree. F., which is leg 23 of curve 21. For temperatures above 73.degree. F., the damper will remain in the fully open position as the actuator piston continues to move outwardly against an overtravel mechanism, as indicated by broken line 24a. When the temperature drops to 73.degree. F., the damper remain fully open until the temperature reaches 71.5.degree. F. (i.e., leg 24 of the curve). As the temperature drops from 71.5.degree. F. to 70.degree. F., the damper begins to close, as shown by leg 25 of curve 21. As will be seen, therefore, there is a hysteresis effect in displacement vs. temperature curve 21 of a typical thermally-powered sensor/actuator's response, which effect always opposes a reversal in motion.
In many applications the thermally-powered sensor/actuator assembly hysteresis effect can be tolerated, but in some applications it would be preferable to be able to tailor or modify displacement vs. temperature curve 21 in order to optimize actuator performance. For example, the hysteresis effect could be substantially eliminated by shortening or eliminating legs 22 and 24 of curve 21. Moreover, the sensitivity of the thermal actuator also could be advantageously changed. Curve legs 23 and 25, for example, might be made to be near vertical, so that full displacement of the actuator piston would occur over a very small room temperature, or process variable, change, for example, 0.2 degrees, rather than 1.5 degrees. It is desirable in many applications, for example, to be able to control room temperature to within about 0.5.degree. F. or less.
Still further, the temperature at which the diffuser actuator opens or closes the damper could advantageously be modified or controlled without changing the actuator wax or adjusting the mechanical linkage between the actuator and the displaceable diffuser damper.
Another source of hysteresis in thermally-powered, VAV, diffuser assemblies is the mechanical linkage between the thermal actuator and the movable damper. When a reversal of the direction of displacement of the damper occurs, for example, the cumulative tolerance and friction effects in the diffuser linkage assembly can produce a lag before diffuser damper displacement occurs.
Still another source of performance affecting factors in thermally-powered diffusers is the positioning or location in the diffuser of the sensor/actuator element. In most thermally-powered, VAV diffusers, the thermally-powered actuator also acts as a temperature sensor. Thus, the sensor/actuator of a thermally-powered diffuser is typically positioned in a flow path or channel in the diffuser, through which room air is induced to flow. As the room air flows past the sensor/actuator, the displaceable piston moves, as shown in FIG. 1. The room air sensor/actuator, however, also "sees," or is influenced by, the heating or cooling air in the supply duct. Thus, the supply air temperature is conducted and radiated in varying amounts throughout the diffuser, which is typically made of formed sheet metal components. The temperature sensed by the "room air" sensor/actuator, therefore, is really a combination of the room air temperature, as induced to flow through a diffuser, and the heating or cooling effects of the supply air. The positioning or placement of the thermal sensor/actuator in the diffuser, that is, the distance between the room air sensor/actuator and supply air flow through the diffuser, will influence the temperature sensed and cause it to vary from the actual room air temperature. This effect can be reduced to some extent by insulation and/or partitioning, and it can be compensated for, to some extent, by selection of the actuator wax.
Still other phenomena will change a thermally-powered diffuser's performance from that which might be theoretically predicted. Small amounts of sensor/actuator waxes will diffuse through pores in rubber seals over long time periods. Moreover, supply air duct pressure will vary over short time periods, causing a given diffuser opening size to discharge more or less supply air for any given control or set point.
Additionally, and very importantly, the thermal load in a room or space can vary substantially depending upon the configuration of the room, the presence of heat-generating equipment, the movement of the sun, the coming and going of occupants. Thus, one room may have a thermal response to heating or cooling which varies substantially from another room of similar size, or which varies substantially over the course of a day.
Still further, the thermal mass from time-to-time and room-to-room may vary and will influence the heating or cooling performance required from a diffuser. Thus, over a weekend, the mass of a room may cool or heat to the ambient temperature, requiring considerable time not only to heat or cool the air in the room, but also to heat or cool the mass of the walls defining the room and the furnishings in the room. Since the furnishings and configurations of rooms can vary, the affect of thermal mass on diffuser performance also can vary from room-to-room.
Supply air pressure differentials also will affect VAV diffuser performance. Compensation in VAV control boxes has been attempted by using pressure sensing equipment and elaborate controls to adjust the VAV box discharge rates. Such compensation techniques can solve upstream problems, but they do not solve the downstream problem of unequal pressure drops between the VAV box and the individual damperless diffusers. The air volume discharged at each downstream, damperless diffuser, therefore, will be unbalanced (unequal) by the differences in pressure drop from the VAV box to the various individual diffusers. This is normally "corrected" through the use of pressure-balancing dampers in the supply air duct proximate each diffuser, but such balancing dampers add to the cost and only "balance" the system for one set of conditions, i.e., when the flow rates change, the system becomes unbalanced again.
Many of the problems above-enumerated in connection with thermally-powered VAV diffusers will apply with equal force to other thermally-powered actuator systems. Thus, when a thermal actuator is used to open and/or close a valve in a fluid system, hysteresis effects, speed of response, actuator positioning (if it also acts as a sensor) and pressure variations in the fluid being controlled by the valve can all influence operation of the system.
Many of these same problems, or analogous problems also exist in VAV diffuser systems which are driven by electric motors. Since motors reverse electrically with little internal friction, hysteresis losses or effects are not significant in electrical motors. Hysteresis-like effects can occur in the diffuser system as a result of a "dead band" in the thermostat controlling the motor. Moreover, gear-based or lever-based damper opening mechanisms coupled to the electrical motor can exhibit hysteresis effects on motion reversals, which are analogous to hysteresis losses in linkages in thermal diffusers. The thermal response differences from room-to-room or time-to-time in a given room are also of concern when motor-driven VAV diffusers are employed, as are supply duct pressure variations. Since the speed of an electric motor response is constant, but it can be either too fast or too slow. Thus, electric motor powered systems also can have problems with constant "hunting" about the set point temperature due to over-responsive or under-responsive performance.
Prior art VAV diffuser or VAV box systems have attempted to address the above-noted problems only in a general manner. There are VAV systems which are known to have one or all three functions of proportional-integral-derivative ("PID") operation control characteristics. See, e.g., "PID Proportional-Integral-Derivative Control," ENGINEERED SYSTEMS, July/August 1987.
Any VAV system in which the response, i.e., damper opening or closing, varies as a function of the input, the sensed temperature, is a "proportional" system. For example, the linear responses of legs 23 and 25 in FIG. 1 show a damper blade displacement which is proportional to the sensed temperature.
Many VAV systems have gone further in that they control damper displacement to reduce the VAV system offset, namely, they attempt to prevent the room air from stabilizing on a temperature which is offset from the set point room air temperature. This is considered to be an "integral" control technique because an integrative term sums the offset error over time and uses the sum as a basis for an additional control signal to reduce the offset and eventually eliminate it.
Finally, some systems also control damper movement so as to reduce thermal overshoot and reduce system hunting, that is, cyclic heating of the room above and below or cyclic cooling of the room below and above, the target or set point room air temperature. This is described as a "derivative" function of the controller in that it anticipates temperature rise or fall as the target temperature is approached and slows the same so as to damp-out thermal overshoot.
When PID controllers for VAV diffuser systems have been employed, they basically operated damper opening in accordance with a transfer function based upon a fixed model which governs the system's performance. Such models are usually established at a test facility in which various "nominal" room physical characteristics are assumed and used to create the desired proportional (P), proportional-integral (PI), or proportional-integral-derivative (PID) diffuser response. All diffusers are then manufactured with the algorithm suitable for the "nominal" room.
Predictably, there are few nominal rooms, and few rooms remain in a "nominal" thermal demand state for very long. It is further known, however, that such P, PI or PID controllers can be manually field-adjusted or "tuned" to accommodate differences in the actual room from those of the test or nominal room. Even this refinement, for example, a tunable PID control system, does not solve the problem of thermal demand variations. When furnishings are added or subtracted from the room, or when occupancy changes, or when equipment is brought into the room, or when rapid changes in sun exposure occur, or as the thermal mass is being brought up or down to the desired temperature, the diffuser operates using the same fixed transfer function between sensed temperatures and resulting damper positions.
Thus, PID systems work well in some spaces, as long as conditions in the space do not change radically, because they tend to have to be tuned to control inside a tight control parameter envelope. If the room conditions fall outside the control envelope, however, PID systems often lose their ability to drive the room temperature back inside the control envelope within a reasonable time period, if at all.
In motor driven VAV systems, PI or PID controllers have simply been achieved using a motor controller having the desired control function output and a temperature sensor such as a thermostat. The motor controller drives the motor in accordance with the PID transfer function in response to the room temperature sensor input.
For thermally-powered VAV diffuser systems, a resistance heater has been mounted to the thermal sensor-actuator containment housing and an electrical controller has been used to control operation of the heater in response to a separate room air temperature sensor. What once was both a sensor and an actuator is now only a thermal actuator and displacement of the movable piston in the actuator is controlled by the heater controller, which again responds based upon a fixed transfer function stored in the controller. An example of a PI system for a thermally-powered, VAV diffuser, is the system manufactured by Titus Division of Tomkins Industries of Richardson, Tex., which system is sold under the trademark Z-COM.
The problem with PI VAV systems can be that the system constantly hunts for the set point as the PI algorithm drives it too fast past the set point in both directions or moves too sluggishly toward the set point. Also PI systems cannot change their response time nor anticipate. Thus, they are best for applications in which the load changes are small and therefore the instability or hunting is small.
While not conventionally the case, refrigerants also could be used, instead of a heater, to produce piston displacement in a thermal actuator.
It is also known in large or sophisticated prior art HVAC systems, to employ controllers which are capable of modifying the performance of the system based upon past experience, i.e., "adaptive" or self-modulating systems. Generally, the cost and complexity of such controllers has made their use with individual VAV diffusers prohibitive. Moreover, such adaptive systems have not been integrated to control thermally-powered sensor/actuators.
As used herein, the expression "adaptive" shall mean the ability of a control system to learn from experience and modify its control behavior in a manner emulating characteristics of the human brain. Generally, such adaptive control systems will take the form of "expert systems," "fuzzy logic systems," "planning systems," "neural networks" or "genetic algorithms," as such terms are used and defined in THE CONTROL HANDBOOK, Sections 57.9 to 57.11, 994-1030 (1996).
Accordingly, it is an object of the present invention to provide a VAV diffuser apparatus and method which are based upon use of a thermally-powered actuator and yet are adaptive so as to enable the control function of the system to be changed over time upon sensing the response of desired ambient parameters.
A further object of the present invention is to provide an adaptive, thermally-powered VAV diffuser which is sufficiently low in cost so that it can be used in every VAV diffuser of a multi-diffuser HVAC system.
Another object of the present invention is to provide an adaptive VAV diffuser and method in which hysteresis effects can be changed or eliminated, the speed of diffuser response can be controlled, and the effect of supply air temperature on diffuser response can be changed or eliminated.
Another object of the present invention is to provide a VAV diffuser in which supply air duct pressure variations can be compensated for so that a pressure-independent VAV diffuser can be achieved.
Another object of the present invention is to provide a diffuser assembly and method having a flow sensing capability so as to allow greater compatibility with large building management HVAC systems.
Still another object of the present invention is to provide a low-cost, adaptive, VAV diffuser and method which is suitable for use with motor-driven diffusers.
Another object of the present invention is to provide a thermal actuator assembly in which hysteresis effects can be controlled or modified to produce a wide range of actuator responses suitable for use in systems other than HVAC diffusers.
Another object of the present invention is to provide a VAV diffuser assembly which can adapt to variations in thermal mass and load.
Another object of the present invention is to provide an adaptive VAV diffuser assembly which is durable, easy to install and adjust in the field, and has interoperability, i.e., can be easily adapted for coupling (plug and play) to complex building management systems.
The VAV diffuser assembly, thermally-powered actuator and method of the present invention have other objects and advantages which will be come apparent from, or are set forth in more detail in, the following Best Mode of Carrying Out the Invention and the accompanying drawing.